batteries - energy from electron transfer

8/7/2019 Batteries - Energy from Electron Transfer

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Batteries -Energy From Electron Transfer


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O n the opposite end of the scale from the power plant is the battery

Personal, portable power supply

But what IS a battery?How does it work?Why can some be recharged and some cant?

Are there alternatives to traditional batteries?Can batteries (or their alternatives) help with the

energy crunch?

Consider the Battery


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A Battery: A system which converts chemicalenergy into electrical energy

More correctly, a battery is an electrochemicalcell :Galvanic Cells convert the energy fromspontaneous chemical reactions intoelectricityElectrolytic Cells use electricity to drive non-spontaneous chemical reactions

Electrochemistry: Some Definitions


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All galvanic cells produce electricity fromreactions which involve the transfer of electrons from one species to another

There are two components to each cell thespecies donating the electrons, and thespecies accepting them

We write half-reactions to represent these twocomponents, and to explicitly show thetransfer of electrons



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The oxidation half-reaction shows the specieswhich is donating electrons

The reduction half-reaction show the specieswhich is receiving electrons

We can also write the net reaction (or overallreaction) for the cell, the balanced sum of thetwo half-reactions

LEO the lion says GER:Loss of Electrons is O xidation; Gain of Electrons is Reduction



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In a nickel-cadmium battery, the reactions looksomething like this:O xidation Cd Cd 2+ + 2 e -

Reduction Ni3+

+ e-

Ni2+

Net Cd + 2 Ni 3+ Cd 2+ +2 Ni 2+

Note: The number of electrons given off in the

oxidation half-reaction must equal thenumber gained in the reduction half-reactionElectrons moving from one place to another

this is electricity


2 x [ ]


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Electrodes are electrical conductors in the cellwhere chemical reactions take placeThe anode is the electrode where oxidationtakes place

The cathode is the electrode where reductiontakes placeThe cathode receives the electrons given off at

the anode and passes them alongThe voltage of the whole cell is the electrical energy

that it gives off, measured in volts (V)The current is the rate at which electrons pass

through the cell, measured in amperes (A)



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Fig.08.p360


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In a nickel-cadmium battery, the reactions actually looklike this:O xidation

Cd(s) + 2 O H- (aq) Cd( O H)2(s) + 2 e -

Reduction2NiO (O H)(s) + 2 H 2O (l) + 2 e - 2Ni( O H)2(s) + 2 O H- (aq)

Net

Cd(s) + 2NiO

(O

H)(s) + 2 H 2O

(l) Cd(O

H)2(s) + 2Ni(O

H)2(s)Note: The number of reactions and the number of

electrons hasnt changed, but were more completely describing the physical and chemical form of the

electrode components

Batteries: The Nickel-Cadmium Battery


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The cell contains a paste of Na O H this provides theO H- ions needed for thereaction, while also providinga medium to pass charge(electrolyte)

The anode consists of solidmetal which is transformedinto cadmium hydroxide

The cathode consists of Ni 3+

ions in a Ni O (O H) pastewhich are transformed intonickel hydroxide


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It is b ecause theproducts of the reactionare solids that the Ni-Cdbattery can be recharged

The solid hydroxides aresticky, cling to the innardsof the battery, and remainin place.

If current is applied, thereaction can be drivenbackwards!


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In a nickel-cadmium battery, we can recharge thebattery by applying an electrical current fromanother source

Cd(s) + 2Ni O (O H)(s) + 2 H 2O (l) Cd( O H)2(s) + 2Ni( O H)2(s)

But most batteries we use arent rechargeable

Why not?

What are the properties of some other typicalbatteries?

Batteries: The Nickel-Cadmium Battery


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Billions upon billions of alkaline batteries areused each year

They are described by size and shape AAA toD

Larger batteries have more stuff, and thus canrun longer

But they all have the same voltage, becausetheyre all based on the sameelectrochemical cell

Batteries: The Alkaline Battery


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But they all have the same voltage, becausetheyre all based on the same electrochemicalcell

O xidationZn(s) + 2 O H- (aq) Zn( O H)2(s) + 2 e -

Reduction2 Mn O

2(s) + H

2O (l) + 2 e - Mn

2O

3(s) + 2 O H- (aq)

NetZn(s) + 2 Mn O 2(s) + H 2O (l) Zn( O H)2(s) + Mn 2O 3(s)

Batteries: The Alkaline Battery


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But the Mn 2O3is not sticky, anddoesnt remainattached to theelectrode. This

battery is notrechargeable


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Lithium-iodine batteries are particularly small and lightweight,but also very long-lived

O ften used in pacemakers, where they can last for 10 years


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Mercury batteries take advantage of the high density of Hg tobe quite small: used in watches, hearing aids, calculators, etc.

Phased out in the 80s due to the toxicity of Hg


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Fig.08.05


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Net:Pb(s) + Pb O 2(s) + H 2S O 4 (aq) 2 PbS O 4 (s) + 2 H 2O (l)

The cathode is made of metallic lead, and the anodeof lead dioxideThe electrolyte is sulfuric acidThis reaction, too, is reversible.

The lead sulfate product clings to the electrodes, soapplied external voltage can reverse the reaction

Batteries: The Lead-Acid Battery


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Lead-acid batteries are referred to as storagebatteries, because this charge-discharge cycle isso reliable

These batteries were used in every automobile untilquite recently

The battery is discharged in order to start the engineO nce the engine is running and burning gasoline, it

turns an alternator which recharges the battery

This process can continue for up to 5 years of normaldrivingAfter that time, enough of the lead sulfate product has

been shaken off the plates that it can no longer

recharge



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Lead-acid batteries are also used in environmentswhere vehicles cannot emit combustion products:Indoor forklifts, golf carts, handicapped carts in

airports, wheelchairs

However, lead is an environmental concern!How do we dispose of the millions and millions of batteries which die each year?

There is a very succesful recycling program in the U.S.

97% of spent batteries are recycledBut environmentally healthier options are under

investigationA leading contender is the magnesium-acid battery


batteries - energy from electron transfer

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